HEDP is commonly used to inhibit corrosion and scale in cooling water systems. It, however, exhibits many properties that are useful in industrial water treatment generally. Thus, it is also used in boiler water treatment, surface cleaning, metal finishing and other industrial operations.
Various processes have been described for producing HEDP. They all, however, suffer from certain disadvantages. The process described in U.S. Pat. No. 3,366,677, for example, uses expensive reagents: phosphorous acid and acetic anhydride.
The process described in U.S. Pat. No. 3,959,360 is difficult to control, and can result in formation of explosive compounds. In that process, PCl.sub.3 is first reacted with glacial acetic acid at a temperature of 60-70.degree. C. to form phosphorous acid and acetyl chloride intermediates which slowly react with each other. Gaseous acetyl chloride and PCl.sub.3 evolve in this step. Because of this, the process requires strictly controlled feeding of both the PCl.sub.3 and acetic acid to maintain the correct hydroxyl balance and to prevent the formation of explosive Lower Oxides Of Phosphorus (LOOP). The next stage involves a long and very difficult to control heating step, in which gaseous acetyl chloride and HCl evolve. Acetic anhydride, which is very expensive, must be added to complete the reaction.
The process of U.S. Pat. No. 4,332,736 improves on that of U.S. Pat. No. 3,959,360, but suffers from serious safety hazards. PCl.sub.3 is first fed into a reactor containing glacial acetic acid at a temperature of above 100.degree. C. At this temperature, reaction intermediates (phosphorous acid and acetyl chloride) react with each other almost instantaneously, forming a mass that separates into two phases. The top phase contains primarily acetic acid, with acetyl chloride. The bottom phase contains primarily acetylated HEDP, with phosphorous acid (&lt;5%) and acetic acid. Under atmospheric pressure and a temperature of &gt;100.degree. C., the acetyl chloride is either reacted to form HEDP, or lost overhead and condensed into an acetyl chloride receiver. Because glacial acetic acid is used, each mole of PCl.sub.3 generates one mole of HCl, and two moles of excess acetyl chloride which must be separated from the HCl and condensed. Extreme caution must be exercised in handling the acetyl chloride because of its toxicity, volatility, combustibility, and reactivity with water and alkalies. Furthermore, because the reaction temperature is high, HCl gas and unreacted acetyl chloride leave the reactor along with entrained PCl.sub.3 and acetic acid. A very large overhead condenser is required to handle this large volume of gases. Also, the entrained PCl.sub.3, and phosphorous acid produced by earlier entrained PCl.sub.3, can react to generate explosive LOOP. In the final step of the process, intermediates of HEDP are hydrolyzed and vacuum stripped (at 10-50 mmHg and 80-120.degree. C.) to remove acetic acid.